Objects at any non-zero temperature radiate electromagnetic energy which can be described either as electromagnetic waves or photons, according to the laws known as Planck's law of radiation, the Stefan-Boltzmann Law, and Wien's displacement law. Wien's displacement law states that the wavelength at which an object radiates the most (λmax) is inversely proportional to the temperature of the object as approximated by the following relation:
            λ      max        ⁡          (              μ        ⁢                                  ⁢        m            )        ≈      3000          T      ⁡              (        K        )            
Hence for objects having a temperature close to room temperature, most of the emitted electromagnetic radiation lies within the infrared region. Due to the presence of CO2, H2O, and other gasses and materials, the earth's atmosphere absorbs electromagnetic radiation having particular wavelengths. Measurements have shown, however, that there are “atmospheric windows” where the absorption is minimal. An example of such a “window” is the 8 μm-12 μm wavelength range. Another window occurs at the wavelength range of 3 μm-5 μm. Typically, objects having a temperature close to room temperature emit radiation close to 10 μm in wavelength. Therefore, electromagnetic radiation emitted by objects close to room temperature is only minimally absorbed by the earth's atmosphere. Accordingly, detection of the presence of objects which are either warmer or cooler than ambient room temperature is readily accomplished by using a detector capable of measuring electromagnetic radiation emitted by such objects.
Two types of electromagnetic radiation detectors are “photon detectors” and “thermal detectors”. Photon detectors detect incident photons by using the energy of said photons to excite charge carriers in a material. The excitation of the material is then detected electronically. Thermal detectors also detect photons. Thermal detectors, however, use the energy of said photons to increase the temperature of a component. By measuring the change in temperature, the intensity of the photons producing the change in temperature can be determined.
In thermal detectors, the temperature change caused by incoming photons can be measured using temperature-dependant resistors (thermistors), the pyroelectric effect, the thermoelectric effect, gas expansion, and other approaches. One advantage of thermal detectors, particularly for long wavelength infrared detection, is that, unlike photon detectors, thermal detectors do not require cryogenic cooling in order to realize an acceptable level of performance.
One type of thermal sensor is known as a “bolometer.” Even though the etymology of the word “bolometer” covers any device used to measure radiation, bolometers are generally understood to be to thermal detectors which rely on a thermistor to detect radiation in the long wavelength infrared window (8 μm-12 μm) or mid-wavelength infrared window (3 μm-5 μm).
The sensitivity of a bolometer generally increases with better thermal isolation of the sensor from its surroundings, with a higher infrared absorption coefficient, higher temperature coefficient of resistance, higher electrical resistance, and a higher bias current. Accordingly, because bolometers must first absorb incident electromagnetic radiation to induce a change in temperature, the efficiency of the absorber in a bolometer relates to the sensitivity and accuracy of the bolometer. Ideally, absorption as close to 100% of incident electromagnetic radiation is desired. In theory, a metal film having a sheet resistance (in Ohms per square) equal to the characteristic impedance of free space, laying over a dielectric or vacuum gap of optical thickness d will have an absorption coefficient of 100% for electromagnetic radiation of wavelength 4d. The following relation shows the expression of the characteristic impedance (Y) of free space:
  Y  =                    μ        0                    ɛ        0            wherein ∈0 is the vacuum permittivity and μ0 is the vacuum permeability.
The numerical value of the characteristic impedance of free space is close to 377 Ohm. The optical length of the gap is defined as “nd”, where n is the index of refraction of the dielectric, air or vacuum.
In the past, micro-electromechanical systems (MEMS) have proven to be effective solutions in various applications due to the sensitivity, spatial and temporal resolutions, and lower power requirements exhibited by MEMS devices. One such application is as a bolometer. Known bolometers use a supporting material which serves as an absorber and as a mechanical support. Typically, the support material is silicon nitride. A thermally sensitive film is formed on the absorber to be used as a thermistor. The absorber structure with the attached thermistor is anchored to a substrate through suspension legs having high thermal resistance in order for the incident electromagnetic radiation to produce a large increase of temperature on the sensor.
The traditional technique used to micromachine suspended members involves the deposition of the material over a “sacrificial” layer, which is to be eventually removed and which is deposited, e.g., by spin coating or polymer coating using a photoresist. The deposition of the thin-film metal or semiconductor can be done with a variety of techniques including low-pressure chemical vapor deposition (LPCVD), epitaxial growth, thermal oxidation, plasma-enhanced chemical vapor deposition (PECVD), sputtering, and evaporation.
Most of the known bolometers, however, have a generally rectangular absorber. Such absorbers exhibit reduced thermal isolation and low electrical resistance, lowering the responsivity of the device. U.S. patent application Ser. No. 13/975,577, filed Aug. 26, 2013, the entire contents of which are herein incorporated by reference, discloses an infrared sensor with increased sensitivity and efficient thermal absorption. While very effective, the device in the '577 does present some challenges.
One challenge is in the manner of incorporating a reference sensor. When measuring the voltage change of a bolometer subject to an incident infrared radiation with an integrated circuit, it is often beneficial to have reference sensors which do not react in the same way to infrared radiation to allow for a relative measurement. Such a reference sensor can be used to make the readout scheme more immune to process variations or temperature changes such as temperature changes of the substrate.
One approach to providing a reference sensor in known systems is to fabricate a sensor which is thermally shorted to the substrate. The substrate thus exchanges thermal energy with the sensor until the sensor has a temperature equal to the temperature of the substrate and the signal from the reference sensor can be used to correct for various errors. For traditional bolometer designs, which include a membrane suspended from the substrate by beams, a thermally shorted reference sensor is easily fabricated. Specifically, in the typical fabrication process a sensor membrane is released from the substrate by isotropically etching a sacrificial layer underneath the structure. Since the membrane size is significantly larger than the distance of the membrane to the substrate, release holes are included in the membrane to decrease the amount of underetch required to release the devices. The reference sensor is thus easily fabricated simply by omitting the release holes. With an appropriate etch time, the infrared sensors will be released while sacrificial material will remain between the reference sensor and the substrate, thermally shorting the reference sensor.
The known method of fabricating thermally shorted sensors as a reference sensor does not work with the devices disclosed in the '577 application. The serpentine structures disclosed in the '577 application are fabricated using a fast release etch without the requirement of release holes. As a result, the method of forming a reference sensor described above does not work for this improved structure.
It would be beneficial to provide a thermally shorted reference sensor that can be fabricated using the same manufacturing processes used in fabricating fast-release sensors. It would be advantageous if such a reference sensor could be fabricated while providing greater flexibility in the release etch time of the sensors.